Add a cache clear [bucket] command

[patcon]

Forked from #13 (comment)

I'm thinking it would be great to have a command to clear all or individual caches. It might make trouble-shooting quirky states simpler if we could just start with saying "run vagrant cache clear [all]. I'll open a new issue.

[fgrehm]

For the future, there is a placeholder to this on the README already. Whoever signs up for this just make sure that gets updated :)

[fgrehm]

Guys, I'm going to close this as it seems there's not much interest on it. I documented how it can be done using plain old bash script over on new docs and I'd be more than happy to look into a PR that encapsulates that into a vagrant command :)